When it comes to analyzing the universe, astronomers are at a distinct disadvantage to almost any other scientist. Geologists can take samples of the Earth and subject it to any number of tests, chemical, physical and otherwise. Archaeologists dig up ruins and examine them carefully. Biologists can dissect plants and animals to see what makes them tick. But astronomers can only look. And what they are looking at isn’t even the way things are today; they are looking into the past. Because of the finite speed of light and the immense distances to stars and galaxies, the light (when I use the term “light” I mean to include all wavelengths in the electromagnetic spectrum, including radio waves, microwaves and x-rays as well as visible light) that reaches Earth started out thousands or millions of years ago. As long as we are fixed on Earth we’re constrained to looking backwards in time. We will never see things as they are.
In order to understand various theories on the age of the universe it’s necessary to understand a little about how physicists used light to measure distances and ages of stars and galaxies.
Ancient man knew that thunder and lightning occur at the same time but there is a lapse of time between the lightning flash and the sound of thunder. He therefore concluded that sound travels slower than light. How much slower and, more importantly, what was the speed of light, remained unknown. Galileo proposed an experiment where two people with lanterns stood some distance apart. One would uncover his lantern and as soon as the other saw the light he would uncover his. By knowing how far apart the two people were and measuring the elapsed time, the speed of light could be calculated. Unfortunately Galileo was in exile and blind by then and was never able to conduct his experiment. A quarter century after his death the experiment was done but due to the real speed of light all that could be determined was that its speed is somewhere between 10,000 km/sec. and infinity.
A few years later a Danish astronomer, Ole Romer, was able to determine that, though high, the speed of light is finite. He did this by scrupulously observing the position of Io, one of Jupiter’s moons. Romer assumed that Io circled Jupiter in a regular orbit. However sometimes it appeared earlier than expected and sometimes later. Romer realized that it appeared later when Earth and Jupiter were on opposite sides of the sun and earlier when both were on the same side of the sun. When Jupiter and Earth were on opposite sides of the sun the light from Io would take longer to arrive at Earth than when they were on the same side of the sun because of the additional distance only if light has a finite speed. After three years of work Romer came up with an estimate of 190,000 km/sec (118,750 miles/second). The exact speed is closer to 300,000 km/sec (187,500 miles/sec) but the important point is Romer established that the speed of light is finite.
The next big question didn’t take long to arise. How is light propagated? Light was assumed to be some sort of a wave, but waves need something to move through. For example, water waves move through water. Sound waves move through air, water, solids and a lot of other media. It had been shown that if an alarm clock was put in a bell jar and the air was sucked out, resulting in a vacuum, there was no sound. Thus sound, like water waves, needs a medium through which to travel. But light could also travel through a vacuum. So how was light transmitted?
To answer this question, like millions of other people, scientists made an assumption, one that, like millions of assumptions made by ordinary people, turned out to be unfounded. They assumed that space is filled with a luminiferous ether or simply ether. The ether was supposed to be something that could vibrate 40 million million times per second yet had almost no density. It was fully transparent, frictionless, and inert. It was everywhere and yet nowhere because no one had ever seen it. In retrospect it sounds a bit like some descriptions of God.
It would take a couple hundred years before the United States’ first Nobel Laureate in physics, Albert Michelson, proved that the ether doesn’t exist while trying to prove that it does. But that’s the subject of another post.